In a shadow mask type cathode-ray tube for producing a color image, a plurality of convergent electron beams are projected through a multi-apertured color selection shadow mask to a cathodoluminescent screen. The beam paths through the mask are such that each beam impinges upon and excites only one kind of color-emitting phosphor element on the screen. Generally, the shadow mask is attached to a rigid peripheral frame, which, in turn, is suspended within the tube envelope.
When a color cathode-ray tube is operated, the electrons that strike the shadow mask cause it to heat up. Since the periphery of the shadow mask is attached to a somewhat heavy frame, the frame acts as a heat sink, and a temperature differential develops between the center and peripheral portions of the mask. Because of the temperature differentials, the mask center, the mask periphery and the frame expand at different rates. This difference in expansion rates causes a doming of certain portions of the mask toward the screen. In the center of the screen, doming causes little effect on the register between the electron beams and phosphor elements, because the straight line projection of the beams to the elements remains unchanged with changes in mask-to-screen spacing. Since the periphery of the mask is fixed to a frame, there is no doming at the mask periphery. Therefore, maximum misregister caused by doming occurs approximately halfway between the mask center and the mask periphery. Misregister is defined as being the amount an electron beam is off-center from its respective phosphor element. Because of doming, the electron beams passing through the mask misregister with the phosphor elements of the screen. The misregister effect of doming peaks after 3 to 5 minutes of initial tube operation, but continues to have a diminishing effect on tube performance for an additional 10 to 15 minutes Once the tube temperatures reach steady state, electron beam misregister caused by expansion of the mask is corrected by temperature-sensitive frame supports which move the mask-frame assembly toward the screen.
A partial solution to the doming problem was disclosed in U.S. Pat. No. 4,136,300, issued to A. M. Morrell on Jan. 23, 1979. The Morrell patent teaches that, if a shadow mask is given greater curvature than that suggested by the prior art, the effects of doming are reduced. In order to maintain proper spacings of phosphor screen lines, the Morrell patent teaches that the horizontal spacing between the shadow mask aperture columns should be varied approximately according to the equation, q=La/3s, where: "q" is the mask-to-screen spacing, "L" is the distance from the deflection plane to the screen, "a" is the center-to-center spacing between aperture columns, and "s" is the center-to-center beam spacing at the plane of deflection.
There may be situations where it is either inconvenient or inappropriate to vary the aperture column-to-column spacing as taught in the above-identified Morrell patent. The system of the present invention provides an alternative which permits use of a shadow mask having substantially greater curvature than its associated faceplate.